MIDDLE MIOCENE PRECIPITATION SEASONALITY IN DERBYSHIRE, UK BASED ON STABLE CARBON ISOTOPE PROFILES IN FOSSIL WOOD

The Brassington Formation is one of the few terrestrial Miocene successions located in the UK. Mummified specimens of conifer wood are preserved in these strata, and a high-resolution stable carbon isotope analysis across their annual growth rings can reveal seasonal patterns in paleoprecipitation. The Brassington Formation has proven to be an important lithostratigraphic unit in understanding the development of Miocene vegetation affected by North Atlantic currents and hypothesized atmospheric circulation changes during the climate cooling that took place during the Middle to Late Miocene. Previous studies on two pollen assemblages of the Kenslow Member in the Brassington Fm. show a shift in the palaeoenvironment from a subtropical conifer-dominated forest (oldest assemblage) to a subtropical mixed forest (youngest assemblage), caused by changes in precipitation seasonality. Although the pollen-based precipitation proxies suggest a shift in rainfall patterns, the exact summer to winter precipitation ratio has remained unknown. Using δ¹³C values collected from Kenslow Member fossil wood, preliminary data shows a mean summer to winter precipitation ratio of 3.7 with a standard deviation of 0.95, along with median total summer and winter precipitation values of 967 mm and 267 mm respectively, with both having a standard deviation of 55 mm. This ratio suggests a precipitation regime that is dominated by summer rainfall with drier winters, in stark contrast to evenly distributed annual rainfall today.